1. Technical Field
The present invention pertains to a valve assembly and method for increasing efficiency thereof, in the neutral mode of operation, without impairing the performance in non-neutral operating modes. The present invention further relates to a hydraulic system that includes the noted valve assembly and an improved method of operation. Specifically, a separate charge pump relief valve is eliminated and the valve bypass orifices utilize increased cross-sectional areas to permit the passage of substantially the full flow of the charge pump at a low restriction to flow through these orifices.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Hydrostatic transmissions have many uses, including the propelling of vehicles, such as grass mowing machines, and offer a stepless control of the machine's speed. A typical hydrostatic transmission system includes a variable displacement main pump coupled in a closed hydraulic circuit with a fixed displacement hydraulic motor. For most applications, the main pump is driven by a prime mover, at a predetermined speed, in a given direction. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the coupled motor. Main pump outflow can be reversed, thus reversing the directional rotation of the motor. In a vehicle, the motor is connected directly, or via suitable gearing, to the vehicle's wheels or tracks. Both acceleration and deceleration of the transmission are controlled by varying the displacement of the main pump from its neutral position. A charge pump is added to the hydraulic circuit in order to charge the closed circuit with hydraulic fluid, through check valves, thus making up for possible lost fluid due to internal leakage. Additional valves, such as high pressure relief valves, bypass valves and hot oil shuttle valves, for example, are also often utilized, in a manner well known in the art. The present invention relates specifically to the hydraulic main pump and motor combination having improved integrated valves for providing smoother operation, particularly during the acceleration phase of the transmission, near its neutral position.
In hydrostatic transmission applications, an over-center variable displacement main pump is normally utilized, with a control handle enabling the operator to control the direction and amount of flow from the main pump. By pushing the handle in one direction, the main pump delivers flow in one direction of motor operation. By pulling the handle in the opposite direction, the main pump delivers flow for the opposite direction. In order to avoid a rough, jerky start of the motor, the prior art has utilized an orifice with a fixed diameter that is added to the closed-loop circuit to increase the width of the dead band of the hydrostatic transmission. The dead band of a hydrostatic transmission refers to the non-response range of the transmission, near its neutral position, where the motor will not turn over due to internal cross-port leakage across the bypass orifice.
Prior art U.S. Pat. No. 6,837,047 B2, also assigned to the assignee of the present invention, and which will be more fully discussed in the “Detailed Description of the Invention”, sets forth a hydraulic valve assembly, as well as a hydraulic system that utilizes this valve assembly, together with a method for increasing the width of the transmission dead band, wherein the bypass orifices are enabled in the neutral position, but are substantially disabled in non-neutral positions. While this has improved transmission performance, the present invention represents an improvement over these prior art constructions by eliminating the previously-required prior art charge pump relief valve and modifying the hot oil shuttle valve by increasing the sizes of the bypass orifices so as to allow the passage of substantially the full flow of the charge pump, at a low pressure drop, i.e., at a low restriction to flow through these bypass orifices, thus resulting in a less costly and more efficient hydrostatic transmission that also operates at a lower working temperature.
The patent literature sets forth a large number of hydrostatic transmission pump/motor systems, including, for example: U.S. Pat. No. 2,961,829 to Weisenbach; U.S. Pat. No. U.S. Pat. No. 3,326,049 to Reinke; U.S. Pat. No. 3,734,225 to Kobald et al.; U.S. Pat. No. 5,211,015 to Schroeder, and U.S. Pat. No. 6,263,670 B1 to Gluck et al. However, none of these prior art structures pertains to the specific structure, system, and method of operation of the present invention.